Meso High Wind Event March 25, 2010

A review of unusually strong winds associated with a meso-high pressure system affecting Northeast Florida on March 25, 2010

On Thursday evening of March 25, 2010, a pre-frontal squall line of showers and a few embedded thunderstorms pushed through Northeast Florida. Trailing stratiform rains occurred behind the line of showers and thunderstorms for approximately 60 miles (Fig 1 ). About 1 to 2 hours after the showers and thunderstorms pushed through, several surface observations indicated strengthening winds from the south and southeast sustained at 20 to 25 mph with gusts of 40 to near 50 mph. One would think that as the showers and thunderstorms pushed through from west to east that winds would turn more westerly. In this special case however, a meso-high pressure system developed behind the squall line. This meso-high can be seen in the graphic and can be tracked going from west to east across the area (Fig 2).

So what caused the anomaly of strong winds behind the line of showers and thunderstorms? Based on the 8 pm March 25, 2010 Jacksonville upper air sounding (Fig 3), there is a fairly dry sub-cloud layer that would likely result in significant cooling of the air mass after the rain and thunderstorms occur. This rain-cooled air would likely create a meso-high pressure system and in this case the high pressure was 1014 mb and migrated southeastward. To the northwest of the meso-high, the low pressure trough associated with the cold front was also approaching and causing a very tight pressure gradient to develop. These unbalanced pressure perturbations and the transient nature of the high causes rapid increases in wind due to intense pressure gradients. The 3 hour pressure change graphic ending at 1 am Friday indicated a significant decrease of around 6 mb centered just south of the Jacksonville Metropolitan area (Fig 4). A pressure trace of Jacksonville and Craig Airport's Automated Surface Observing Systems also showed a significant drop of about 5 to 6 millibars in just 2 to 3 hours (Fig 5).

Meso-highs typically form right behind organized mesoscale convective systems, such as squall lines, and move quickly causing patterns of strong convergence and divergence. As a result, strong winds may follow behind squall lines and may cause wind damage, in certain cases, even an hour or two after the heaviest rain has ended. Fortunately, in this case, surface winds did not appear to exceed 50 mph based on observational data but minor damage did occur in the Gainesville area when a tree fell onto two cars and knocked over a light post.